User groups allowed fields problem

[waidis]

Hello,
I'm using TYPOlight 2.8 and now setting up user groups but when it comes to set up allowed fields on a list of tl_page can't do changes on these fields:

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When pressing save button getting error message like this:

Forbidden
You don't have permission to access /typolight/main.php on this server.

Any suggestion?

[Ben]

Are you logged in with an admin account when you're trying to do this? Is this a fresh install or an upgrade?